Surfactant proteins are important functional components of pulmonary surfactant, a complex lipoprotein that acts to reduce lung alveolar surface tension. Expression of the genes encoding surfactant proteins, SP-A, SP-B, SP-C, and SP-D is developmentally regulated in fetal lung tissue. Prematurely born infants can develop Respiratory Distress Syndrome (RDS), due to a lack of adequate surfactant synthesis. RDS is the leading cause of neonatal morbidity and mortality in developed countries. Treatment of infants and pregnant mothers with glucocorticoids accelerates lung maturity, decreasing the risk of RDS. Surfactant protein B (SP-B) is a critical component in the function of surfactant. Surfactant that is deficient in SP-B protein results respiratory failure in full term infants, and surfactant that contains reduced levels of SP-B protein is not effective in the ability to reduce lung alveolar surface tension. Glucocorticoids alter surfactant protein expression in fetal human lung via both transcriptional activation and by altering the stability of surfactant protein mRNA. In particular, glucocorticoids increase both the transcriptional expression and the stability of SP-B mRNA. Since glucocorticoids are used clinically in treatment of premature infants, it is important to understand the molecular mechanism(s) by which glucocorticoids act to regulate surfactant protein gene expression in type II epithelial cells in fetal lung. The objective of the proposed research is to define the molecular mechanism(s) by which glucocorticoids stabilize human SP-B mRNA while destabilizing SP- A mRNA. The following specific aims are proposed: (1) functionally localize the region(s) in the SP-B mRNA that mediates regulation of human SP-B mRNA stability by glucocorticoids in vivo; (2) characterize the in vitro mRNA:protein interactions at the cis-acting element(s) that mediate regulation of human SP-B mRNA stability by glucocorticoids; and (3) identify the protein(s) that mediates regulation of human SP-B mRNA stability and study its regulation. The goal of this research is to define the molecular mechanisms whereby glucocorticoids enhance SP-B gene expression and reduce SP-A gene expression by post-transcriptional mechanisms during fetal lung development and, therefore provide insight into the design of treatment regimens that increase the effectiveness of antenatal glucocorticoids in the enhancement of surfactant synthesis by the fetal lung and prevent RDS.